This invention relates generally to power supplies and more particularly to a regulated voltage power supply in which the output voltage is electrically isolated from the source of power. In regulated voltage power supplies it is necessary to convey information about the status of the regulated output voltage to a power controller. This information is commonly referred to as an error voltage signal or simply an error signal. If it is desired to electrically isolate the regulated output voltage from a source of power to which the power controller is also connected, then it is necessary that the error signal be conveyed across an electrical isolation boundary.
Two devices common in the prior art and used for conveying an error signal across an electrical isolation boundary are known as an optical isolator or opto-coupler and a transformer. The opto-coupler requires that the error signal be converted to a current and then back to a voltage on the other side of the isolation boundary. The transformer used in the prior art to convey an error signal across an electrical isolation boundary is most commonly implemented as a pulse transformer, whereby the error voltage signal controls the magnitude of a voltage that is regularly impressed across one winding of the transformer and derived again from another winding. This process is known as pulse amplitude modulation (PAM).
A typical prior art circuit for deriving an error signal and conveying it via an opto-coupler to a power controller is shown in FIG. 1. In the electronics industry, the device which generates the error signal is commonly labeled a TL431 and is referred to as an adjustable shunt regulator or an adjustable zener. In the application being discussed, its primary advantage over an operational amplifier is that the reference voltage used to derive the error signal is built in. The opto-coupler can be described to have a driven side and an output side. The driven side is the LED portion of the opto-coupler and is also called an opto-LED. The output side is the light-transistor. In the prior art circuit of FIG. 1, the driven side of the opto-coupler OC is connected in series with the cathode terminal of an adjustable zener diode TL431 and a current limiting resistor RG. Usually, a second resistor RB is added in parallel with the series connection of the driven side of the opto-coupler OC and the current limiting resistor RG. The purpose of the second resistor RB is to provide a pre-load for the adjustable zener diode TL431 when the current through the series connection of the driven side of the opto-coupler OC and the current limiting resistor RG is less than the minimum current of typically 1 mA required for stable operation of the adjustable zener diode TL431. The error signal to be transferred through the isolation boundary of the converter is generated by dividing the voltage appearing between the + and - output terminals of the converter with the help of resistors RU and RL and comparing it against the internally generated reference voltage of the adjustable zener diode TL431. When the fraction of the output voltage of the converter appearing across the resistor RL becomes higher than the internal reference voltage of the adjustable zener diode TL431, the voltage between the cathode and anode of the adjustable zener diode TL431 begins to decrease. The result is that the voltage across the series connection of the driven side of the opto-coupler OC and the current limiting resistor RG increases, leading to increased current in the opto-LED within the opto-coupler OC. The increased current in the opto-LED causes an increase in the opto-transistor, leading to an increase in the voltage across the terminating resistor RV that is connected in series with the output side of the opto-coupler OC. Accordingly, an increase in the output voltage of the converter leads to an increase in the voltage across the terminating resistor RV, which is located at the other side of the isolation boundary. A compensating capacitor CFB is required in feedback systems for ensuring stability of the feedback loop. Instead of a single capacitor, more sophisticated compensating networks are also feasible.
There are two significant disadvantages associated with the prior art opto-coupler described above. In the PAM process, an ON pulse is terminated by monitoring when a PAM current has increased to a predetermined and controllable level. An ON pulse is initiated by monitoring when the voltage across a clamp diode has decayed below its normal forward bias level, indicating a commensurate decay in PAM current, following termination of the previous ON pulse.